Hi.
The most important thing in designing a CMFB circuit is its gain and speed. In some applications the speed is more important, but if it is not, usually after a short transient response your CMFB would work properly. However there are two main types of CMFB : Continuous-Time and Discrete-Time CMFBs. Chosing one of these depends on your application. In folded cascode structure, gain is not very important often, but don't forget! it really depends on your closed-form structure, for closed-loop part will act as a positive feedback for CM signal. So the CMFB loop should have a larger gain than this positive feedback loop in order to compensate it. There are lots of good and simple CMFB structures in electronic books. But I recommend Johns and Martin's book and Razavi's book about Analog IC design.
Good luck.
Regards,
EZT